When conditions are favourable, rakali reproduction is well-suited to producing numerous juveniles to colonise any available vacant habitat. The animals can potentially breed throughout the year (Olsen 1982; Gardner and Serena 1995), though litters most commonly appear in spring and summer in the cooler southern parts of the range (McNally 1960; Olsen 1982; Gardner and Serena 1995; Leigh and Breed 2020). The length of pregnancy is only about 5 weeks, lasting between 33 and 41 days in captivity (Olsen 1982). A female rakali only has four nipples and most typically raises 3 (Olsen 1982) or 4-5 (McNally 1960) babies at a time, but litters of up to seven babies have been recorded both in captivity (Olsen 1982) and the wild (McNally 1960).
After giving birth, a mother will suckle her offspring for about a month (Olsen 1982), with 1 or 2 litters often produced annually by a wild female (McNally 1960). By comparison, captive females typically give birth to 2 or 3 (up to a maximum of 5) litters annually (Olsen 1982). Although females raised in captivity may be able to breed when just 4 months old, most do not mature before the age of 8 months (Olsen 1982). In practice, most wild females probably don’t start breeding until they are more than one year old (McNally 1960). Females become reproductively senescent (are no longer capable of breeding) when about 3.5 years old (Olsen 1982).
Photo courtesy of David Fleay
LITERATURE CITED
Gardner JL and Serena M (1995) Observations on activity patterns, population and den characteristics of the water rat Hydromys chrysogaster along Badger Creek, Victoria. Australian Mammalogy 18, 710-75.
Leigh CM and Breed WG (2020) A demographic study of the water-rat (Hydromys chrysogaster) on the River Torrens in Adelaide, South Australia. Australian Mammalogy 42, 277-282.
Olsen PD (1982) Reproductive biology and development of the water rat, Hydromys chrysogaster, in captivity. Australian Wildlife Research 9, 39-53.